Reimplement file permissions for images and pipelines

Currently, tiamat-ng only allows displaying image data from it's own working directory. However, this does not work from within docker context, as mounting the directories into the container breaks symlinks (which the files are likely to be with git-annex/datalad). Solution: Reimplement an explicit mapping, which replaces specific paths ("aliases") with the correct filesystem paths.

Same goes for URLs: It should be possible to pass a json as a URL, from a specific server list.